Electronic tremulant



March 13, 1951 F. M. SCHMIDT ELECTRONIC TREMULANT Filed June 1. 1949l/mrf f INVENTOR. firzczaflimdf BY W Patented Mar. 13, 1951 UNITEDSTATES PATENT OFFICE ELECTRONIC TREMULAN T Francis M. Schmidt, NorthTonawanda, N. Y., assignor to The Rudolph Wurlitzer Company, NorthTonawanda, N. Y., a corporation of Ohio Application June 1, 1949, SerialNo. 96,545

. 11 Claims.

This invention relates to musical instruments and more particularly toan electronic tremulant for use in an electronic musical instrument.

- It is often desirable in musical instruments, particularly electronicorgans, to have a tremulant efiect producing a fluctuating intensity orpitch for desirable musical tones.

Prior tremulants in electronic musical instruments have generally falleninto one of two classes, either mechanical or electrical. The mechanicalsystems have generally employed a slowly rotating fan mounted adjacentto and directly in front of the output translator or speaker. Typicalelectronic tremulants have utilized a low frequency oscillator tomodulate a sound amplifier stage or to modulate the tone generator.Mechanical systems are generally unsatisfactory in that they require toofrequent attention, are difficult to control and are inclined to benoisy. Prior electronic tremulants have often been unsatisfactory inthat they presented an undesirable time lag in starting and ending,required control of high voltages, or caused undesirable interactions inthe electronic circuits.

An object of this invention is to present an electronic tremulant whichdoes not require the direct control of high voltages.

. A further object of this invention is an electronic tremulant which issubstantially instantaneous in starting and ending operations.

. Another object of this invention is to produce an electronictremulant, the operation of which is not affected by connection intocircuit with the amplifying stages of an electronic musical in-.

strument.

Another object of this invention is to produce an electronictremulanthaving a controllable degree of tremulancy.

, Other and further objects and advantages of the current invention willbe apparent from the following description when taken in conjunctionwith the accompanying drawings in which:

. Fig. 1 is a perspective view of the console of an electric organ inwhich the electronic tremulant may be used; 7

Fig. 2 is a schematic wiring diagram of the electronic tremulant; and

Fig. 3 illustrates the relative variation in volume and pitch of thesound amplifier output when the tremulant is used.

' A console I of an electronic organ in which the electronic tremulantwhich is the subject of this invention may be used is shown in Fig. l.

The console has one or more banks of keys 3 to beactuated by hand andmay also have a. pedal keyboard 5 to be actuated by the musician's feet.Foot pedals 'l are also provided for varying the volume and may also beused to control the de-'. gree of tremulancy if desired. A row of stoptablets ii is also provided for selecting tone generators, and one ormore of these stops may control the operation of the tremulant. Anelectronic tremulant is used to shunt the input of one of the amplifierstages. circuit of one stage which may amplify a musical tone producedin any desirable manner, such as the vibrating reed capacitor explainedby Hoschke in his Patent 2,015,014, is shown comprising a vacuum tube Hhaving a plate load resistor and a source of positive plate potentialshown as a battery l5. An isolating resistor I! is provided to preventthe tremulant from having any effect on the amplifier stage having thetube H. The signal from the previous stage passes through the isolatingresistor I! to the next stage including a coupling capacitor l9, grid.

resistor 2l,.vacuum tube 23 and cathode resistor 25. The output circuitof the tube 23 may be conventional and is not shown as the electronictremulant operates most particularly in conjunction with the inputcircuit.

The electronic tremulant includes a low frequency oscillator 2'! whichmay be of any desirable type operating at a sub-audible frequency. 6.4cycles per second is recommended as producing a pleasing result. The lowfrequency oscillator is coupled as by direct coupling to a.

limiter 29 which may be of any convenient conventional type which limitsthe amplitude of the output of the low frequency oscillator so that.

the tremulant effect may be readily controlled by the musician With norandom variations. The limiter also provides some degree of shaping tothe output wave of the generator and acts as a buifer to preventunstable operation of the os-' cillator under load. One of the outputterminals of the limiter is grounded and the other is coupled to thecontrol grid 33 of a voltage amplifier tube 35 which may be deemed acontrol tube. Plate potential for the plate 31 of control tube 35 isvsupplied through a plate load resistor 39 from a convenient source ofpositive potential which is grounded at its negative end as at 4!.Potential for the screen grid 43 is supplied through a screen gridresistor 45 from the same source of positive potential. The suppressorgrid 41 is directly connected to the cathode 49 which is in turnconnected to ground through a pair of series resistors 5| and 53, thelatter of which is shunted by a switch 55. The cathode 49 is also Theoutput.

connected through a resistor 51 to the positive source of potential anda capacitor 59 shunts both of the resistors i and 53 to ground. Theresistors 5!, 53 and 51 constitute a voltage divider providing cathodebias for the control tube 35. With the switch 55 closed, the controltube 35 is biassed at its normal operating potential, but when theswitch 55 is open, the: additional bias developed across the resistor 53causes the tube 35 to cut off and become inoperative.

The control grid 33 is connected to ground through a series of resistorsBI, 63, 55 and 61 with the output of the limiter connected between theresistors 6i and 53. Tap switch contacts 69 and H are connectedrespectively between the resistors 63 and 65 and 55 and El. A third tapswitch contact 73 is not connected and a grounded switch arm '15 may beselectively connected with any one of these three tap switches toprovide a means for determining the magnitude of the output of thelimiter 29 which is applied to the control grid of the control tube 35.The switch arm may be controlled by one of the foot pedals l or by oneor more of the stop tablets 9. The normal input capacitance of thecontrol tube 35 is augmented by a capacitor ll connected between thecontrol grid 33 and the plate 37. The control tube 35 is coupled to theinput of the amplifier tube. 23 by means of a capacitor '19 connectedbetween the junction of the resistor SI and capacitor TI and theresistor H and capacitor [9.

As the control grid potential is changed relative to the cathode by theoutput from the oscillator 21 and the limiter the input capacitance ofthe control tube varies. The changing of input capacitance in responseto varying control grid potential is well known to those in the art andthe function of an electronic tube as a voltage controlled capacitanceis described in the Patent 2,017,270 to Yolles. During a positive halfcycle of output from the limiter the control tube bias decreases, whichincreases the input capacitance, thus increasing the shunting effect ofthe electronic tremulant on the. input to the amplifier tube 23. Thisshunting effect causes the response of the amplifier to be consequentlylowered, particularly at the higher frequencies, giving the curve shownat A in Fig. 3. During the negative portion of the cycle the controlgrid bias increases and the input capacitance decreases, thus decreasingthe shunting efiect of the tremulant and giving the curve shown at B inFig. 3. The relative variation in frequency response is shown by theshaded area in Fig. 3 with the normal response curve of the soundamplifier being at a point midway between the extremes shown.

The low frequency oscillator may be operated at all times when theelectronic musical instrument is operating. With the switch open thetube 35 is out oh. and the oscillator has no effect upon the operationof the electronic musical instrument. When the switch 55 is closed, asby means of one of the stop tablets 9, the tube 35 immediately becomesoperative and as the oscillator has been running continuously, theshunting effect on the sound amplifier is instantaneous so that there isno time lag between the time the tremulant switch is actuated and thetremulant effect is noticed in the output. Likewise, the tremulant isstopped instantaneously by opening the switch 55 to out off the controltube 35. The limiter prevents reflected impedances from impairing theoperation of the low frequency oscillator 21 and the capacitor 59prevents any clicks from being apparent when the switch 55 is opened orclosed. As was noted before, the position of the tap switch arm 15controls the potential from the limiter applied to the control grid 33and consequently changes the variation in input capacity of the tube 35so that the magnitude of the tremulant may be controlled by themusician.

Various changes may be made in the apparatus chosen for illustrationwithout departing from the spirit and scope of the appended claims, andI therefore intend to be limited only by the following claims.

I claim:

1'. An electronic musical instrument including an electronic amplifierstage and an electronic tremulant, said tremulant comprising anelectronic tube, an input circuit for said electronic tube and connectedsubstantially in shunt with said electronic amplifier stage, and meansfor varying the input capacitance of said electronic tube whereby toshunt a variable portion of a musical tone passed by said electronicamplifier stage to produce a tremulant eifect.

2. An electronic tremulant as defined in claim 1 in which the means forvarying the input capacitance of the electronic tube includes means forvarying its grid potential.

3. An electronic musical instrument including a plurality of electronicamplifier stages and an electronic tremulant, said tremulant comprisinga variable impedanc connected between. two amplifier stages andsubstantially in shunt there with to shunt a portion of a musical tonebeing amplified to ground, and means for varying said impedanceperiodically thereby to produce a tremulant effect in said musical tone.

4. An electronic tremulant as defined in claim 3 in which the variableimpedance comprises substantially a variable capacitance.

5. An electronic musical instrument including a plurality of electronicamplifier stages, an electronic tube, an input circuit for said tubecon.- nected between two amplifier stages and substantially in shunttherewith, and means for varying the input capacitance of saidelectronic tube whereby to shunt a variable portion of a musical signalto ground to produce a tremulant effect in said musical signal.

6. An electronic tremulant as defined in claim 5 in which the means forvarying the input ca.- pacitance of the electronic tube includes meansfor varying the grid potential of said tube.

7. An electronic musical instrument including an amplifier stage and, anelectronic tremulant connected substantially in shunt to vary theresponse of said amplifier stage, said electronic tremulant comprisingan electroni tube having a control grid, a low frequency oscillator,means interconnecting said oscillator and the grid of said electronic.tube whereby to change the input capacitance of said electronic tube,and a network connecting the control grid of said elec'- tronic tubesubstantially in shunt with said amplifier stage to produce a tremulanteffect in the musical tones of said musical instrument.

8. An electronic tremulant as defined in claim '7 in which the lowfrequency oscillator operates at a sub-audible frequency.

9. An electronic musical instrument comprising an amplifier stage. andan electronic tremulant, said tremulant comprising an electronic tubehaving a control element, electrical circuit elements connecting saidcontrol element of said electronic tube in circuit with said amplifierstage, a low frequency oscillator, and a limiter interconnecting saidoscillator and said electronic tube, said oscillator and said limitersupplying oscillations devoid of random variation in amplitude to thecontrol element of said electronic tube with no reaction on said lowfrequency oscillator to vary the input capacitance of said electronictube and thereby introduce a tremulant efiect into the circuit in whichthe electronic tube is connected.

10. An electronic musical instrument including amplifying means and anelectronic tremulant, said tremulant comprising an electronic tubehaving a control grid, means connecting said tube in circuit with saidamplifying means to vary the amplifying characteristics thereof, acontinually operating low frequency generator, circuit means connectingthe output of said low frequency oscillator to the control grid of saidelectronic tube, means for normally biassing said electronic tube beyondcutofl, and means for bias- REFERENCES CITED The following, referencesare of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,301,871 Hanert Nov. 10, 19422,322,884 Roetken June 29, 1943

